While fluoxetine is frequently prescribed to autistic patients in clinical practice, its efficacy, side effects, maintenance and discontinuation effects have not been definitively established. Mixed results of efficacy, particularly between pre- and post-pubertal patients, and a lack of well-controlled trials leave many gaps in our knowledge about SSRIs, such as fluoxetine, in autism. Furthermore, it is important to determine whether specific subtypes of individuals (those with increased repetitive behaviors), or those with specific genetic variants or polymorphisms of the serotonin system respond differentially to fluoxetine. Little controlled data is available with medications that target the repetitive behavior domain of autism, which contributes to substantial functional impairment. This study will compare liquid fluoxetine vs. placebo in treating child/adolescent autistic patients and assess the relationship between dose, blood levels and response to treatment. The relationship between candidate genes of the serotonin system, symptom improvement, and side effects will be examined. The study will consist of three treatment phases: Phase 1 - a 12-week doubleblind placebo-controlled parallel acute treatment phase, Phase 2 - a 12-week treatment extension phase, and Phase 3 - a 12-week double-blind placebo-controlled discontinuation phase. We aim to evaluate a targeted treatment of a core symptom domain in a stratified sample of autistic children with repetitive behavior, and explore the relationship between genetic variation of the 5-HT system and fluoxetine treatment response and side effects.